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Submitted on: 19 July 2009.
Revised on: 29 August 2009.
Accepted on: 11 October 2009.
___________________Economics of Transportation---ELSEVIER
Submitted on: 21 August 2004.
Revised on: 10 December 2004.
Accepted on: 07 February 2005.
___________________Economics of Transportation---ELSEVIER
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